White adipose tissue (WAT) is specialized for the storage of triacylglycerol (TAG) to release fatty acids (FA) into the circulation for other organs to use as an energy source. In contrast, brown adipose tissue (BAT) uses FA to activate uncoupling protein 1 (UCP1) for non-shivering thermogenesis to dissipate energy as heat. In human infants, there is an abundance of BAT that eventually converts into WAT with age. However, the recent discovery of presence of functional BAT in adults has generated renewed interest in the study of BAT for potential prevention and treatment of obesity. Nevertheless, the process and underlying mechanisms for the maintenance of BAT phenotype or the conversion of BAT to WAT are not understood. Here, we present preliminary data that show a previously unknown role for desnutrin, the major adipose TAG hydrolase in BAT phenotype and function. Namely, adipose-specific ablation of desnutrin in mice causes a striking conversion of BAT to a WAT-like tissue, repressing UCP-1 and other BAT-enriched genes and changing mitochondrial morphology, severely impairing thermogenesis. In contrast, overexpression of desnutrin in adipose tissue increases UCP1 expression and thermogenesis. To explain these observations, we hypothesize that desnutrin- catalyzed lipolysis is required for maintaining BAT phenotype and function. Our studies will elucidate the mechanisms underlying adipose tissue plasticity reflected in the interconversion of BAT and WAT and the signaling pathway for desnutrin's essential role in maintaining BAT phenotype and function. The three specific aims proposed are: 1. to determine the requirement of desnutrin-catalyzed lipolysis for maintaining BAT phenotype and function. 2. To examine the role of AMP-activated protein kinase in regulating BAT function through phosphorylation and activation of desnutrin. 3. To study PPARa as a downstream effector of desnutrin-catalyzed lipolysis for maintaining BAT phenotype and function. These studies will clearly demonstrate the critical role that desnutrin-catalyzed lipolysis plays in the maintaining the function and phenotype of BAT. This research will highlight the involvement of AMP-activated protein kinase in activating desnutrin, as well as the participation of PPARa as a downstream target of desnutrin-catalyzed lipolysis, in maintaining functional BAT. Our findings may provide underlying mechanisms for the well-documented increase in adaptive thermogenic capacity upon cold exposure. Understanding this process may provide future therapeutic targets to control obesity by maintenance or induction of BAT in adults.